This Script by Marijan Tompa is very handy if you work for someone who puts the ALT text for Images in the XML: Headline or somewhere else in the metadata of an image. Instead of applying the ALT text to each individual image through Object Export Options, you can run this script to do it for you.

Note: When I tried to run this in CS6, the Script did not work unless I moved it from the Users folder to the Application folder. See forum post, All InDesign Scripts won’t work for further information on troubleshooting this problem.